The newly discovered BiS$_2$-based LaO$_{1-x}$F$_{x}$BiS$_2$ ($x$=0.5) becomes superconductive at $T_c$=2.5 K. Electrical resistivity and magnetization measurements are performed under pressure to determine the pressure dependence of the superconducting transition temperature $T_c$. We observe that $T_c$ abruptly increases from 2.5 K to 10.7 K at a pressure of 0.7 GPa. According to high-pressure X-ray diffraction measurements, a structural phase transition from a tetragonal phase ($P$4/$nmm$) to a monoclinic phase ($P$2$_1/m$) also occurs at around $\sim$ 1 GPa. We consider that a pressure-induced enhancement of superconductivity is caused by the structural phase transition.

Pressure-induced Enhancement of Superconductivity in BiS$_2$-layered LaO$_{1-x}$F$_x$BiS$_2$

Superconductivity (SC) and charge-density wave (CDW) are two contrasting yet relevant collective electronic states, which have received sustained interest for decades. Here, we report that, in a layered europium bismuth sulfofluoride, ${\mathrm{EuBiS}}_{2}\mathrm{F}$, a CDW-like transition occurs at 280 K, below which SC emerges at 0.3 K, without any extrinsic doping. The Eu ions were found to exhibit an anomalously temperature-independent mixed valence of about +2.2, associated with the formation of a possible dynamic CDW. The mixed valence of Eu gives rise to self electron doping into the conduction bands mainly consisting of the in-plane $\mathrm{Bi}6p$ states, which in turn brings about the CDW and SC. In particular, the electronic specific-heat coefficient is enhanced by $\ensuremath{\sim}50$ times, owing to the significant hybridizations between $\mathrm{Eu}4f$ and $\mathrm{Bi}6p$ electrons, as verified by band-structure calculations. Thus ${\mathrm{EuBiS}}_{2}\mathrm{F}$ manifests itself as an unprecedented material that simultaneously accommodates SC, CDW, and $f$-electron valence instability.

Possible charge-density wave, superconductivity, and f-electron valence instability in EuBiS 2 F

BiS2-based LaO1−xFxBiS2 (x = 0.5) becomes superconductive at Tc = 2.5 K. Electrical resistivity and magnetization measurements are performed under pressure to determine the pressure dependence of the superconducting transition temperature Tc up to 18 GPa. We observe that Tc abruptly increases from 2.5 to 10.7 K at a pressure of 0.7 GPa. According to high-pressure X-ray diffraction measurements of LaO1−xFxBiS2 (x = 0 and 0.5), a structural phase transition from tetragonal to monoclinic also occurs at ∼0.8 GPa. We consider that the pressure-induced enhancement of superconductivity for LaO1−xFxBiS2 is caused by the structural phase transition.

Pressure-Induced Enhancement of Superconductivity and Structural Transition in BiS 2 -Layered LaO 1- x F x BiS 2

Very recently a new family of layered materials, containing BiS2 planes was discovered to be superconducting at temperatures up to Tc=10 K, raising questions about the mechanism of superconductivity in these systems. Here, we present state-of-the-art first principles calculations that directly address this question and reveal several surprising findings. The parent compound LaOBiS2 possesses anharmonic ferroelectric soft phonons at the zone center with a rather large polarization of $\\approx 10 \\mu C/cm^2$, which is comparable to the well-known ferroelectric BiFeO3. Upon electron doping, new unstable phonon branches appear along the entire line Q=(q,q,0), causing Bi/S atoms to order in a one-dimensional charge density wave (CDW). We find that BiS2 is a strong electron-phonon coupled superconductor in the vicinity of competing ferroelectric and CDW phases. Our results suggest new directions to tune the balance between these phases and increase Tc in this new class of materials.

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Ferroelectric Soft Phonons, Charge Density Wave Instability, and Strong Electron-Phonon Coupling in BiS$_2$-Layered Superconductors

Review of superconductivity in BiS 2 -based layered materials

We report a strategy to induce superconductivity in the BiS$_2$-based compound LaOBiS$_2$ Instead of substituting F for O, we increase the charge-carrier density (electron dope) via substitution of tetravalent Th$^{+4}$, Hf$^{+4}$, Zr$^{+4}$, and Ti$^{+4}$ for trivalent La$^{+3}$ It is found that both the LaOBiS$_2$ and ThOBiS$_2$ parent compounds are bad metals and that superconductivity is induced by electron doping with \emph{T$_c$} values of up to 285 K The superconducting and normal states were characterized by electrical resistivity, magnetic susceptibility, and heat capacity measurements We also demonstrate that reducing the charge-carrier density (hole doping) via substitution of divalent Sr$^{+2}$ for La$^{+3}$ does not induce superconductivity

S. Spagna

Papers

Superconductivity induced by electron doping in the system La1-xMxOBiS2 (M = Ti, Zr, Hf, Th)